The European Patent Application 0 114 085 which was published on July 25, 1984 corresponding to U.S. Pat. No. 4,615,569, discloses a dimensionally stable anode for an aluminum production cell which anode comprises a substrate of a ceramic, a metal or other materials which is coated with a layer of a cerium oxycompound. The anode is stable under conditions found in an aluminum production cell, provided that a sufficient content of cerium is maintained in the electrolyte.
The anode as described in the above European Patent Application performs well with respect to dimensional stability, however, contamination of the aluminum by substrate components may occur under certain circumstances. As shown by microphotographs, the cerium-containing coating is in general comprises of a non-homogeneous structure leaving small interstices between coated areas, which provide access of the electrolyte to the substrate. In such cases, the electrolyte may corrode the substrate leading to a limited but undesired contamination of the aluminum by substrate components.
The French patent application 2 407 277 discloses a method of electrolyzing chlorides of e.g. magnesium, sodium, calcium or aluminum in electrolytes having temperatures between 500.degree.-800.degree. C. using an anode comprising a substrate and a coating of an oxide of a noble metal, whereby a certain concentration of an oxide or oxychloride of a metal which is more basic than the metal produced is maintained in the bath. Thus, by increasing the basicity of the bath the solubility of the anode coating is reduced.
This method provides better stability of the anode coating by the addition of melt additives. It relates to the stabilization and protection of the anode coating and not of the substrate as is one of the hereunder defined objects of the present invention. In the above patent application the substrate itself is stable in the chloride bath at the given operating temperature and is essentially protected by the coating.
In contrast, in a molten cyrolite bath at e.g. 960.degree. C. an imperfect coating or substrate may not simply be protected against corrosion by modifying the basicity of the bath as described in the French patent but is unstable and corrodes. In a cyrolite bath, a mere modification of the basicity would not improve the stability of the substrate as it does with a coating of an oxide of a noble metal which is essentially stable in the described chloride bath of FR 2 407 277.